Ukip has confirmed that its policy on immigration, one of its strongest selling points at the polls, is "under review". A party spokesman also accepted that it may increase the cap it has previously backed of 50,000 people moving to the UK a year.

Ukip in its 2010 manifesto said: "We aspire to ensure that any future immigration does not exceed 50,000 people per annum."

All three mainstream parties are promising to put Ukip policy under renewed scrutiny. Michael Gove, the education secretary, promised there would be civilised scrutiny, adding that Ukip had a black hole in its spending commitments of £120bn. Eric Pickles, the communities secretary, claimed Ukip was already looking uncomfortable on specifics of its immigration policy.

The Ukip leader, Nigel Farage, on Friday continued to offer a broad-brush analysis, saying that in the past 10 years there had been more migrants into Britain than between 1066 and 1950.

Ukip's manifesto on its website is more specific, saying that at present immigration is adding 1 million people to the population every four to five years.

The outline policy includes a promise to introduce a five-year freeze on immigration for permanent settlement until UK borders are under control. The party has said there would be exceptions, but has not yet said how many. It has also said over-staying on a visa would become a criminal offence. Immigrants would not be able to apply for public housing or benefits until they had paid tax for five years.

It adds: "Any future immigration for permanent settlement must be on a strictly limited and controlled basis where that can clearly be shown to benefit the British people as a whole and our economy." No definition of this is provided.

Ukip also says it would "enable people to come and work in the UK by means of a points-based work permit system for limited periods of time and to fulfil specific gaps in the job market that cannot be filled by the existing work force". No detail is given.

It offers a form of amnesty, saying "EU citizens who have been established in the UK for seven years or more will, depending on their circumstances, be able to apply for permanent leave to remain (provided they fulfil certain criteria and are eligible to apply for work permits)".

Ukip also adds it would leave the European convention of human rights and the European convention on refugees.